摘要
A series of marginal-sea basins and fault-depression basins were formed in eastern China under the background of subduction of the West Pacific plate.Different types of helium-rich natural gas reservoirs(He>1000 ppm,1 ppm=1μmol mol^(-1))have been found in these basins:helium-rich CO_(2)gas reservoirs,helium-rich N_(2)gas reservoirs,and helium-rich hydrocarbon gas reservoirs.Based on the analysis of gas geochemical data,the source and accumulation mechanism of helium in these heliumrich natural gas reservoirs were discussed.Helium-rich natural gas has relatively high 3He/4He ratios(0.88-4.91 Ra,average 2.82 Ra).The ^(3)He/^(4)He ratio characteristics of mantle xenoliths and mantle-derived CO_(2)gas reservoirs indicate that the helium in these helium-rich natural gas reservoirs is mainly mantle-derived(>70%).The original mantle volatile is mainly CO_(2)with a low helium concentration(He<200 ppm),and the enrichment of mantle-derived helium in the gas reservoir is mainly related to the dissolution and mineralization of CO_(2).During this process,the CO_(2)/3He ratio decreases from 2×10^(9)to approximately 2×10^(6).As CO_(2)dissolves and mineralizes,the concentration of conservative gases(He and N_(2))increases in the remaining CO_(2)gas proportionally to the loss of CO_(2).Large amounts of carbonate minerals,such as dawsonite,which are relatively enriched in 13C,are found in CO_(2)reservoirs in eastern China.The relative enrichment of^(12)C in residual CO_(2)gas is important evidence of the dissolution and mineralization of CO_(2).The relative abundance of mantle-derived helium and N_(2)gas increases thousands of times during the dissolution and mineralization of CO_(2),which is the main accumulation mechanism of mantle-derived helium-rich CO_(2)gas reservoirs and helium-rich N_(2)gas reservoirs.Helium-rich gas from the mantle is mixed with alkane gas generated by organic matter in the sedimentary basin to form helium-rich hydrocarbon gas reservoirs.
基金
supported by the National Key R&D Program of China(Grant No.2021YFA0719002)
the National Natural Science Foundation of China(Grant Nos.42141021 and 42141022).
